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Found the issue. Semi-loose fuel line connection at the fuel pump. It was sucking air at higher rpm's. FYI for all who are reading. Truck runs excellent now.

On mine, I am still confused. It has full power most of the time, then loses power - truck barely moves 10 mph - then back to normal. Again, changed air filters and fuel recently. Each time it does it I change the fuel filter. A Cummins mechanic told me to clean the leads on the fuel shutoff...

Yes, black smoke when it is not moving like it should. When it is running normal everything is fine, when it loses power it smokes like a train - and will not rev up above 1000 rpms - then it clears and screams down the haul road again. Never thought about a clogged muffler.

Is there an adjustment on the fuel pump? My M917 is acting up in a similar way, except it has a lot of power - then a little - then a lot - then a little. I can't make sense of it. Replaced all filters, checked lines, etc.